Igneous Rocks

Igneous Rocks

(or magmatic rocks), rocks formed from molten magma as it solidifies and crystallizes. Based on solidification conditions, two principal types of igneous rocks are distinguished: effusive (volcanic, extrusive), which have solidified on the surface of the earth after magma has poured forth in the form of lava during volcanic eruptions, and intrusive (abyssal), which have solidified in a stratum of the earth’s crust between other rocks. Owing to their rapid solidification, effusive rocks are usually fine-grained and consist partially (sometimes completely) of glass. Larger crystals called phenocrysts are often found in them, giving rise to what is known as “porphyritic texture.” Intrusive rocks, which solidify slowly deep within the crust, have a holo-crystalline and more coarse-grained structure.

Rocks of the normal series and the alkalic series (alkali granites, nepheline syenites, phonolites) are distinguished on the basis of the alkali content of each group of igneous rocks. The alkali silicates (aegirines, alkali amphiboles, and feldspathoids) are placed in the alkalic series.

Various minerals are also associated with different types of igneous rocks. For example, tin, tungsten, and gold are associated with acidic igneous rocks; titanomagnetite and copper with basic igneous rocks; chromium, platinum, nickel, and other minerals with ultrabasic igneous rocks; and titanium, phosphorus, apatites, zirconium, rare earths, and other minerals with alkali igneous rocks.

Igneous rocks can be used as building materials (Artik tuffs and labradoritites, for example), abrasive materials (pumice), and thermal insulation materials (pumice, perlite). In addition, they can serve as raw material for the extraction of valuable components (for example, aluminum from nepheline syenites) and as foundations for hydraulic and other structures.

Petrofabric, the study of the preferred alignment of minerals and rocks, is used as a proxy measurement for paleocurrent directions in sedimentary rocks, emplacement flow directions in igneous rocks, and strain directions in deformed and metamorphosed rocks.

All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional.